Historically, sash locks for double hung windows have served several purposes. The first sash locks served only to prevent the windows from being opened, the traditional function of a lock. The sash lock later evolved to serve two other purposes, to draw the check rails of the upper and lower sashes together tightly, and to a limited extent, force the upper sash in an upwards direction and the lower sash in a downwards direction. Both of these functions serve to make the window more weather-tight.